dt-bindings: mailbox: add binding for Microchip IPC mailbox controller

Add a dt-binding for the Microchip Inter-Processor Communication (IPC)
mailbox controller.

Signed-off-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>